On 26 April 1986, reactor 4 of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukraine, exploded. With dozens of direct casualties and thousands of health complications stemming from the disaster, it is one of only two nuclear accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident, which happened in Japan. The response involved more than 500,000 personnel and cost an estimated 18 billion rubles. It remains the worst nuclear disaster and the most expensive disaster in history, with an estimated cost of US$700 billion. The disaster occurred during a test to simulate the cooling of the reactor during a serious accident in blackout conditions. The operators carried out the test following an accidental drop in reactor power. Upon shutting down the reactor in such conditions, pervasive design flaws led to a power surge. The reactor components ruptured and lost coolant, and the resulting steam explosions and meltdown destroyed the reactor building. This was followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe. The Soviet government established a 10 kilometres (6.2 mi) exclusion zone 36 hours after the accident, initially evacuating around 49,000 people. This was later expanded to 30 kilometres (19 mi), resulting in the evacuation of approximately 68,000 more people. The government did not publicly acknowledge the disaster until two days after the explosion, when elevated radiation levels were detected in Sweden.